Digital printers capable of outputting variable data which are in use and include those based on a inkjet method, and methods using magnetism, ions, electric condensation, etc, in addition to an electrophotographic method, but the electrophotographic method is currently the most widely spread. This electrophotographic method is a technique used in copying machines and laser printers, and also called a xerography method. This is a variable printing method which allows rewriting every time, and has been creating new demand for printing.
In a digital printer using this electrophotographic method, a positive charge is given by a corona discharge to a photo conductor drum charged by laser, and if an image is described into this photo conductor drum by the laser or a light emitting diode (LED), the charge is neutralized in the portion of the drum where the image has been described. If toner is provided to this part, the toner only adheres to the part where the charge remains, thereby forming an image. Then, the printing machine is used to transfer the image by superposing paper on the toner image.
This electrophotographic method described above further includes a direct transfer method which performs the transfer from the photo conductor drum directly to the paper, and an offset transfer method in which the image is once transferred to an intermediate transfer sheet and then transferred from the intermediate transfer sheet to the paper. The former provides printing quality lower than that of normal offset printing, and is not capable of printing on an embossed sheet and the like. The latter is very expensive because the intermediate transfer sheet has a particular configuration and performance. The latter also has a particular installation structure in which an electrode, among others, has to be removed when the intermediate transfer sheet is attached to a transfer drum, which causes much difficulty in handling.
The intermediate transfer sheet for use in the latter method includes, for example, an intermediate transfer blanket described in Japanese Patent Publication Laid-open No. 11-512910. This blanket comprises an image transfer portion adapted to receive an image which has already been formed, and a body portion attached to the transfer drum. The image transfer portion comprises an alignment layer provided under a release layer to be a transfer surface, while the body portion comprises a conductive top layer, a compressive layer and woven cloth layer. The blanket is formed by stacking the alignment layer of the image transfer portion on the top layer with or without the conductive layer in between.
To use the intermediate transfer blanket having such a configuration, an elongate conductive bar in which a series of L-shaped attachment legs is integrally formed is attached to an end of the intermediate transfer blanket for installation on the drum. To attach the conductive bar, the conductive layer is directly inserted without including the release layer, the alignment layer and an obstacle layer, thereby integrally forming the conductive bar.
Thus, in the known blanket, the conductive bar serves as the electrode to supply a voltage to the conductive layer. Therefore, the electrode also has to be removed when the blanket is attached to the transfer drum, which causes a problem in that the structure is complicated and that the attachment is troublesome. Another problem is that when the blanket is replaced, it is necessary to cut the blanket along an edge of an attachment member which is the conductive bar and to separate the attachment member from the blanket in order to remove the attachment member from the drum. Its manufacturing method is also complicated and significantly expensive.
It is therefore an object of this invention to provide an image transfer sheet which solve the foregoing problems, wherein printing quality equal to that of the normal offset printing is maintained in printing with an image forming technique (apparatus) using a principle of the electrophotographic method, and wherein the electrode can be directly removed from the drum and can be installed on the drum in a significantly simple manner.
Furthermore, this invention provides an image transfer sheet which can be manufactured in an inexpensive, simple and easy manner.
It is another object of this invention to provide an intermediate image transfer sheet particularly suited to transfer a liquid toner image.